In a computer, data storage is the place where data is held in an electromagnetic or optical form for access by a computer processor.

Storage is frequently used to describe the devices and data connected to the computer through input/output (I/O) operations -- that is, hard disk and tape systems and other forms of storage that don't include computer memory and other in-computer storage. For the enterprise, the options for this kind of storage are of a much greater variety and expense than those related to memory.

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Storage hierarchy

Enterprise data storage is often classified as primary and secondary storage, depending on how the data is used and what type of media it is placed on.

Primary storage holds data in memory (sometimes called random access memory or RAM) and other built-in devices, such as the processor's L1 cache. Secondary storage commonly includes data on hard disks, tapes and other devices requiring I/O operations. Secondary storage media is often used in cloud storage.

Primary storage is much faster to access than secondary storage because of the proximity of the storage to the processor and the nature of the storage devices. On the other hand, secondary storage can hold much more data than primary storage.

Antiquated terms for primary and secondary storage are main storage and auxiliary storage, respectively. To add to the confusion, there is an additional meaning for primary storage that distinguishes actively used storage from backup storage.

The main types in use today include hard disk drives (HDDs), optical storage and solid-state storage. Spinning HDDs use platters stacked on top of each other coated in magnetic media with disk heads that read and write data to the media. HDDs are the most commonly used storage in personal computers, servers and enterprise storage systems, but are rapidly giving way to faster solid-state drives (SSDs).

SSDs store data on nonvolatile flash memory chips. Unlike spinning disk drives, SSDs have no moving parts. They are increasingly found in all types of computers, although they remain much more expensive than HDDs.

Measuring storage

Bits and bytes are the basic measurements for computer storage. One single binary value (1 or 0) makes up a bit, and eight bits make up one byte. Other capacity measurements --and their abbreviations -- to know are:

It's still rare for a single storage system or connected system to reach an EB of data, but there are now storage systems built to store PBs.

Capacity requirements

Data storage capacity requirements tell users how much storage is needed to run an application, a set of applications or data sets. Capacity requirements take into account the types of data. For instance, simple documents may only require KBs of capacity, while graphic-intensive files, such as digital photographs, may take up MBs, and a video file can require GBs of storage. Computer applications commonly list minimum and recommended capacity requirements to run.

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